I think you misunderstand what For( does, this command is for loops and doesn't support 'intervals' if intervals of time is what you mean, you can however use a For( as a delay in a program
the For( commands needs three arguments:

For( variable , start , end, (optional) increment )

An example:

For(A,1,10)
<Code here repeats 10 times>
End

Vikt0r

Another way to make a delay is using the rand command, although its effectiveness varies from calculator to calculator (i.e., rand(52 might be too long on your calculator, while it is just right on mine).

To mention, I actually think that in the Texas Instrument official guide, they say that the second number submitted to the for loop is the increment, but obviously that is wrong. That's where the for loop starts. I've read through the programming part of the guide several times, but I've never seen anything in there about a fourth argument in a for loop. Some other stuff they don't mention:
As a final argument for a circle( command, if you put an imaginary number in a list (say Circle(0,0,9,{i})) then the circle is drawn several times as fast.
If you put a "-1" as the first argument in a text( command, text is drawn in the homescreen style font
You don't need a newline after delvar commands.
The sub( command can be used to divide a number by 100
There is an undocumented "%" command that also divides a number by 100